Nevertheless, when I refresh the log in page and try to log in again, it works flawlessly. And even sometimes it works at first, straight away. I'm really having a hard time trying to figure how could this happen, how could it stop working so... randomly.

Well, I talked to the Network Administrator and he tried to use another user to access the LDAP base. Apparently it worked fine. Don't know if the former user had some restrictions, but now it seems to work. However I'm not sure yet, even with the bunch of tests we did yesterday... But thank you for your readiness in helping me.